1. The Field of the Invention
The present invention relates to methods for measuring the surface area of a semiconductor wafer and, more specifically, to methods for measuring the surface area of a semiconductor wafer having a varied topography.
2. The Relevant Technology
Integrated circuits are manufactured by an elaborate process in which a variety of different electronic devices are integrally formed on a small silicon wafer. Conventional electronic devices include capacitors, resistors, transistors, diodes, and the like. In advanced manufacturing of integrated circuits, hundreds of thousands of electronic devices are formed on a top region of a single silicon wafer.
The variety of electronic devices making up an integrated circuit are formed through a series of layering and stripping steps. By selectively combining layers of conductive, non-conductive, and semi-conductive material, each of the different electronic devices can be formed. As the different electronic devices are formed, the topography of the silicon wafer changes. That is, at select times in the fabrication process, the integrated circuit may comprise a variety of different trenches, plateaus, contact holes, islands, or other structures having a varied height and size. As these different structures are formed, the overall surface area of the silicon wafer changes.
The ability to determine the surface area of a silicon wafer at any given time in the fabrication process is becoming more important as the size of integrated circuits become smaller and more dense. For example, one of the processes used for depositing a layer of material over a silicon wafer is chemical vapor deposition (CVD). In one CVD method, a selected material is reacted in a furnace to form molecules in a vapor state. A gas is then used to transfer the molecules down a long heated tube where the silicon wafers are stored. As the gas passes by the silicon wafers, the molecules are deposited onto the surface area of the silicon wafer until a film of a desired thickness is obtained.
One problem encountered in this CVD process is that as the molecules within the gas are deposited on the first wafers in the tube, the concentration of molecules in the gas decreases. In turn, this decreases the rate of molecular deposition on subsequent wafers. As such, the thickness of the deposited material varies along the length of the tube. This problem is referred to as depletion.
By knowing the surface area of each wafer, it is possible to determine the rate at which the molecules are deposited on the silicon wafers and thus depleted from the gas. Knowing this information, variables such as the gas flow rate and temperature along the tube can be altered to produce a more uniform deposition on the silicon wafers.
Knowing the surface area of a silicon wafer is also useful in determining the properties of a capacitor. The capacitance or effectiveness of a capacitor is dependent on the surface area of the capacitor. In one use, the surface area of the capacitor is roughened so as to increase the surface area of the capacitor without increasing the overall surface area on the silicon wafer that the capacitor uses. The ability to know the total surface area of a silicon wafer is useful in determining the actual surface area and thus the capacitance of each capacitor.
Ascertaining the surface area of a silicon wafer is also useful in determining the depth of a trench. Trenches are formed during the fabrication of integrated circuits. It is often important to know the depth of a trench so that subsequent steps can be properly performed. Historically, the depth of a trench has been determined by inserting a probe within the trench. Today, however, trenches are becoming increasingly small, which makes the probe insertion depth determining technique increasingly difficult. Further, the probe insertion technique may be inadequate for determining all dimensions across the topography of a silicon wafer necessary to accurately determining the surface area of the silicon wafer. Importantly, knowledge of the topography of the structures on the silicon wafer are integral to determining the total surface area of the silicon wafer.
Determining the total surface area of a silicon wafer can also be used in monitoring quality control. For example, by comparing the theoretical surface area of a silicon wafer to the actual surface area of the silicon wafer, the determination can be made as to whether there are any significant defects on the silicon wafer.